Broadcasting, whether for entertainment or other purposes, requires significant amounts of land for transmitters, towers, and/or antennas. The towers and/or antennas may be guyed, which can add to the size of an installation. In many locations (or “markets”) around the world, broadcasters have pooled resources to consolidate land use to small numbers of facilities with complex apparatus. This strategy is recognized and regulated by agencies, such as the Federal Communications Commission (FCC) in the U.S., which have oversight regarding the broadcast characteristics of each signal as well as issues such as interaction between signals. In recent years, the addition of digital broadcasting to the previous and continuing analog broadcasting has made this situation still more complex.
Within entertainment broadcasting, a distinction may be drawn between television (TV), both very-high-frequency (VHF) and ultra-high-frequency (UHF), on the one hand, and radio, both medium-frequency (MF) amplitude modulated (AM) and very-high-frequency (VHF) frequency-modulated (FM). A basis for distinction is bandwidth of each signal. Where TV is assigned channels that are 6 MHz wide (in the U.S.) and have multiple subcarriers operating in synchrony within the channel, radio channels are 20 kHz wide for AM and extend to +/−200 kHz for FM, and each operate with a single carrier. Digital broadcasting for TV and for AM and FM radio uses a variety of mechanisms to interoperate with analog substantially free of interference.
Radio, thanks to its narrower bandwidth, is capable of being received with lower incident signal energy than TV, and thus typically achieves greater range for a given transmitter power level. The FM band is assigned between the low and high VHF TV assignments. Digital broadcasting for TV is scheduled as of this filing to complete replacement of analog within a few years, in concert with shutting down existing VHF TV channels and reassigning this bandwidth to new users. FM radio broadcasting is not so circumscribed; digital and analog signals are scheduled to coexist indefinitely, and the present VHF band assignment for FM is expected to remain unchanged.
As noted, because of high demand for programming differentiation and for other reasons, many FM stations, particularly those in high-demand regional markets, pool resources, which resources in various instances may include one or more of transmitters, high-power signal transmission lines, antenna tower structures, and antennas. Certain pooling strategies for multi-station FM sites are relatively simple, while others have proven to be challenging.
Historically, some multi-station FM sites have successfully used multi-station combiners to combine several separate high-power station signals (i.e., signal outputs from separate transmitters) onto a common transmission line. The combined signal can then be fed into an appropriate broadband antenna to be radiated into free space. Multi-station combiners of known types have typically relied on combiner techniques such as branch combiners and multiple Constant Impedance Filters (CIFs). The combiners that can be built from such component parts—each component being large and expensive—are demonstrably successful, and have significant benefits, but have limitations that suggest that other solutions to the challenge of developing cost-effective and reliable FM radio service may be worthy of consideration.
What is needed in the art is a combiner technology for multi-station FM sites that achieves performance at least comparable to that of known systems while representing negligible technical risk and offering a much-reduced cost. Were such needs met, extension of the technology beyond FM radio broadcasting into other areas would also be potentially useful.